Geographic Information Systems and Science SECOND EDITION Paul A. Longley, Michael F. Goodchild,...

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9. GIS Data Collection

Overview

IntroductionPrimary data captureSecondary data captureData transferCapturing attribute dataManaging a data capture project

Data Collection

One of most expensive GIS activitiesMany diverse sourcesTwo broad types of collection

Data capture (direct collection)Data transfer

Two broad capture methodsPrimary (direct measurement)Secondary (indirect derivation)

Data Collection Techniques

Raster VectorPrimary Digital remote

sensing imagesGPS measurements

Digital aerial photographs

Survey measurements

Secondary Scanned maps Topographic surveys

DEMs from maps

Toponymy data sets from atlases

Client-server GIS Costs

10 Seats 100 Seats

$ % $ %

Hardware

30 3.4 250 8.6

Software 25 2.8 200 6.9

Data 400 44.7 450 15.5

Staff 440 49.1 2000 69.0

Total 895 100 2900 100

Stages in Data Collection Projects

Planning

Preparation

Digitizing / TransferEditing / Improvement

Evaluation

Primary Data Capture

Capture specifically for GIS useRaster – remote sensing

e.g. SPOT and IKONOS satellites and aerial photographyPassive and active sensors

Resolution is key considerationSpatialSpectralTemporal

Typical Reflectance Signatures

Vector Primary Data Capture

SurveyingLocations of objects determines by angle and distance measurements from known locationsUses expensive field equipment and crewsMost accurate method for large scale, small areas

GPSCollection of satellites used to fix locations on Earth’s surfaceDifferential GPS used to improve accuracy

Total Station

Secondary Geographic Data Capture

Data collected for other purposes can be converted for use in GISRaster conversion

Scanning of maps, aerial photographs, documents, etcImportant scanning parameters are spatial and spectral (bit depth) resolution

Vector Secondary Data Capture

Collection of vector objects from maps, photographs, plans, etc.Digitizing

Manual (table) Heads-up and vectorization

Photogrammetry – the science and technology of making measurements from photographs, etc.COGO – Coordinate Geometry

Scanner

Vector Over Raster

Digitizer

Batch Vectorization

Typology of human errors in digitizing: (A) undershoots and overshoots; (B) invalid polygons; and (C) sliver polygons

Error induced by data cleaning

Mismatches of adjacent spatial data sources that require rubber-sheeting

COGO construction tools used to represent geographic features

Data Transfer

Buy v build is an important questionMany widely distributed sources of GIKey catalogs include

US NSDI Clearinghouse networkGeography Network

Access technologiesTranslationDirect read

Comparison of data access by translation and direct read

Managing Data Capture ProjectsKey principles

Clear plan, adequate resources, appropriate funding, and sufficient time

Fundamental tradeoff between Quality, speed and price

Two strategiesIncremental‘Blitzkrieg’ (all at once)

Alternative resource optionsIn houseSpecialist external agency

Figure 9.17 Relationship between quality, speed, and price in data capture (Source: after Hohl 1998).

SummaryData collection is very expensive, time-consuming, tedious and error proneGood procedures required for large scale collection projectsMain techniques

Primary• Raster – e.g. remote sensing• Vector – e.g. field survey

Secondary• Raster – e.g. scanning• Vector – e.g. table digitizing

Geographic Information Systems and Science SECOND EDITION Paul A. Longley, Michael F. Goodchild,...

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